† Electronic supplementary information (details including: full synthetic methodsingle crystal X-ray structure renement aexperiments; and MM simulations. CCDcrystallographic data in CIF or o10.1039/c4sc03531c
‡ A.G. Slater and Y. Hu contributed equal
Cite this: Chem. Sci., 2015, 6, 1562
Received 14th November 2014Accepted 11th December 2014
Anna G. Slater,‡a Ya Hu,‡b Lixu Yang,a Stephen P. Argent,a William Lewis,a
Matthew O. Blunt*b and Neil R. Champness*a
The synthesis and surface-based self-assembly of thymine-functionalised porphyrins is described. Reaction
of 1-formylphenyl-3-benzoyl-thymine with suitable pyrollic species leads to the formation of tetra-
(phenylthymine)porphyrin (tetra-TP) or mono-thymine-tri-(3,5-di-tert-butylphenyl)porphyrin (mono-TP).
Single crystal X-ray diffraction studies demonstrate the self-association of mono-TP in the solid state
through thymine/thymine hydrogen-bonding interactions but in solution this interaction (Kd ¼ 6.1 �3.0 M�1) is relatively weak in comparison to the heteromolecular interaction between mono-TP and
9-propyladenine (K ¼ 91.8 � 20.5 M�1). STM studies of the tetratopic hydrogen-bonding tecton, tetra-
TP, deposited on an HOPG substrate reveal the formation of an almost perfectly square self-assembled
lattice through thymine/thymine hydrogen-bonding. Co-deposition of tetra-TP with 9-propyladenine
leads to the adoption of preferable thymine/adenine interactions leading to the formation of a
heteromolecular tetra-TP/9-propyladenine hydrogen bonded array including both Watson–Crick
thymine/adenine interactions and adenine/adenine hydrogen-bonding. The studies demonstrate a
pathway for the self-assembly of tetratopic hydrogen-bonding tectons and the use of preferential
heteromolecular thymine/adenine interactions for the disruption of the homomolecular tetra-TP array.
Studies of the self-assembly of tetra-TP and 9-propyladenine demonstrate a strong dependence on
overall concentration and molar ratio of components indicating the importance of kinetic effects in
surface self-assembly processes.
Introduction
The study of self-assembled two-dimensional structures onsurfaces has become an area of intense interest over recentyears.1–5 Particular focus has been applied to the use of inter-molecular interactions in attempts to control the relative orga-nisation of molecules and to create well-dened moleculararrangements. Intermolecular interactions that have beensuccessfully used to prepare self-assembled structures includecoordination bonds,6,7 halogen bonds8,9 and van der Waalsinteractions.10–12 Hydrogen-bonds13–21 have received extensiveattention with a major focus being the exploitation of moleculesbearing multiple carboxylic acids to form extended arrays.13–18 It
ham, University Park, Nottingham, NG7
am.ac.uk
ollege London (UCL), London, WC1H 0AJ,
ESI) available: Additional experimentals and characterization; full details ofnd CIFs; binding measurements; STMC 1034260 and 1034261. For ESI andther electronic format see DOI:
ly to this work.
is also possible to prepare surface-based self-assembled struc-tures from more than one molecular component22 using thewell-established concepts within supramolecular chemistry ofthe molecular tecton23,24 and supramolecular synthon.25 Byusing two or more tectons that are designed such that theyincorporate hydrogen-bonding moieties that favour hetero-molecular interactions multi-component arrays can betargeted.19–21
Perhaps the most widely known heteromolecular synthonsare those formed between DNA nucleobases. Nucleobases havebeen extensively studied in the eld of surface-based self-assembly with a particular focus on understanding theirhydrogen-bonding behaviour. Two recent reviews coveradvances in the area26,27 and illustrate the complexity of suchsystems. Nucleobases have been studied using scanningtunnelling microscopy28–31 including thymine (T)30 and adenine(A),31 of particular relevance to this study. Throughout thesestudies a variety of substrates have been used, includingmetallic surfaces and highly-ordered pyrolitic graphite (HOPG),as well as a variety of conditions, ultra-high vacuum and solid–liquid interfaces. A study of particular interest is that byBesenbacher et al. who describe investigations of combinationsof thymine and adenine.32 As anticipated intermolecularhydrogen-bonding interactions are observed between the
thymine and adenine generating A–T–A–T quartets that involvereverse Hoogsteen hydrogen-bonding. Examples of functional-ized adenine and thymine molecules have been studied inattempts to inuence the nature of the intermolecularhydrogen-bonding interactions. Notable examples, described byBonifazi and co-workers,33,34 report the synthesis of di-uracilrods and their subsequent self-assembly with molecules thatpresent hydrogen-bonding motifs complementary to that ofuracil (and thymine).
In this study we have synthesised thymine-functionalisedporphyrin molecules, or tectons, (Scheme 1) that present theimide hydrogen bondingmoiety of thymine in such amanner toencourage divergent assembly with either other molecules ofthe same type or with a simple propyl-functionalised adeninespecies, 9-propyl-9H-purine-6-ylamine (9-propyladenine).Porphyrins represent useful scaffolds for the basis of hydrogen-bonding tectons as functionalization of each meso-positionleads to a tetratopic tecton. The vast majority of tectons previ-ously investigated in surface self-assembled networks1–5 areeither rod-like ditopic tectons6,13,19–21 or tritopic7,12,14,19–21 tectons.Although tetratopic hydrogen-bonding tectons are known,notably with carboxylic acid groups,15–18 systems with morespecic hydrogen-bonding moieties for heteromolecular arrayshave not been studied previously for surface self-assembly.Porphyrins present a highly attractive target for the basis oftetratopic tectons due to their fourfold symmetry and have beenused for the assembly of a variety of surface-based self-assem-bled structures35 including covalently-coupled arrays36 and,notably with carboxylic acid hydrogen-bonding moietiesthrough the use of 5,10,15,20-tetrakis-(4-carboxylphenyl)-porphyrin.37
Two target porphyrin molecules were identied: tetra-(phenyl-thymine)porphyrin (tetra-TP) for surface-based self-assemblystudies and mono-thymine-tri-(3,5-di-tert-butylphenyl)porphyrin (mono-TP) as a model compound to probe the natureof intermolecular interactions in the solution phase. The basicdesign encompassed functionalisation of the porphyrin corewith phenylthymine groups and the remaining meso positions,in the case of mono-TP, bearing 3,5-di-tert-butylphenyl moie-ties. The tert-butyl functionalised appendages were chosen dueto their ability to enhance porphyrin solubility, inhibiting p–p
interactions, and allowing more facile solution-based studies.The synthesis of a porphyrin functionalised with uracil in themeso-position, 10,15,20-tetrakis(1-butyl-6-uracyl)porphyrin, hasbeen reported previously38 and was found to form nanobrestructures. The design of 10,15,20-tetrakis(1-butyl-6-uracyl)porphyrin is not appropriate for surface self-assembly studiesdue to the orthogonal arrangement of the uracil moiety withrespect to the porphyrin core. Thus we adopted a design thatpositioned a phenyl group between the porphyrin plane and thethymine group so that both porphyrin and thymine could be co-planar and parallel to the surface that the molecules are to bedeposited upon. Our studies demonstrate that it is not onlypossible to prepare homomolecular hydrogen-bonded arrayswith tetra-TP but heteromolecular arrays can be generated bycombination of suitable tectons, in our case tetra-TP and9-propyladenine.
Results and discussionSynthesis and structural studies
Porphyrin molecules were prepared functionalised in the meso-positions by phenyl–thymine moieties. Our approach requiredthe synthesis of 1-formylphenyl-3-benzoyl-thymine from 4-for-mylphenylboronic acid and 3-benzoylthymine39 using aCu(OAc)2-mediated Chan–Lam–Evans-modied Ullmanncondensation reaction to facilitate the cross-coupling process.40
Reaction of 1-formylphenyl-3-benzoyl-thymine with a largeexcess of pyrrole in the presence of InCl3 affords the desireddipyrromethane which can be subsequently used in porphyrinsynthesis.
In the case of mono-TP (Scheme 2) the benzoyl-thyminefunctionalised dipyrromethane was reacted with a suitable tert-
Scheme 2 Synthetic route used for the preparation of mono-TP.
Fig. 1 Single crystal X-ray structures of (a) benzoyl-mono-TP and (b)mono-TP. Note the relative orientation of the thymine moiety withrespect to the porphyrin plane. Hydrogen atoms of the tert-butylgroups are omitted for clarity. Atoms are colored as follows C – lightblue; N – dark blue; H – white; O – red.
Fig. 2 View of the single crystal X-ray structure of mono-TP illus-trating the formation of the inter-thymine R2
2(8) double hydrogen-bonding interaction (represented with the two dotted lines) observedin the solid state. Hydrogen atoms of the tert-butyl groups are omittedfor clarity. Atoms are colored as follows C – light blue; N – dark blue;H – white; O – red.
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butyl functionalised carbinol species, using Lindsey'sapproach,41 in the presence of triuoroacetic acid (TFA) withsubsequent oxidation using 2,3-dichloro-5,6-dicyano-1,4-benzo-quinone (DDQ). Subsequent deprotection of the benzoyl-pro-tected thymine was achieved in good yield using NH4OH as thedeprotecting agent.42 The synthesis of the tetra-TP was morestraightforward (Scheme 3) following direct reaction of 1-for-mylphenyl-3-benzoyl-thymine with a large excess of pyrroleusing TFA to initiate the reaction. Oxidation with DDQ gave thetarget molecule in 11% yield and deprotection, again usingNH4OH,42 afforded tetra-TP in 91% yield.
The products were characterised by conventional techniquesand in the case of mono-TP and its benzoyl-protected precursorby single crystal X-ray diffraction. For both of these compoundsslow diffusion of MeOH into a solution of the compound, eitherCH2Cl2 (mono-TP) or CDCl3 (benzoyl-mono-TP), led to growthof single crystals of suitable quality for X-ray diffraction studies.In both cases the X-ray structure conrms the identity of theproduct and the relative arrangement of the thymine moietywith respect to the porphyrin ring (Fig. 1).
The structure of the mono-TP reveals valuable informationabout the nature of the intermolecular interactions anticipatedfor thymine-substituted porphyrin species (Fig. 2). The mostpertinent feature is the formation of an R2
2(8) double hydrogen-bonding interaction43 between thymine moieties on adjacentmolecules. Each N–H/O hydrogen bond within the inter-thymine R2
2(8) synthon is crystallographically equivalent [N/O¼ 2.818(3) A; H/O ¼ 1.94 A; :N–H/O ¼ 172.0�] and fallswithin the typical range expected for such a hydrogen bond.44
Adjacent molecules pack such that p–p interactions areobserved between the thymine moiety of one molecule and apyrrole of an adjacent molecule (centroid/centroid separationof 3.50 A). As anticipated in both structures, benzoyl-mono-TPand mono-TP, the phenyl ring that links the porphyrin moietywith the thymine group adopts an orientation that approachesorthogonality with respect to the porphyrin plane (71.02�:benzoyl-mono-TP; 64.01�: mono-TP) and the thymineapproaches co-planarity with the porphyrin core in bothinstances (17.21�: benzoyl-mono-TP; 13.37�: mono-TP).
In order to assess the strength of interaction betweenporphyrin-appended thymine moieties and a simple propyl-
Scheme 3 Synthetic route used for the preparation of tetra-TP.
1564 | Chem. Sci., 2015, 6, 1562–1569
functionalised adenine species, 9-propyl-9H-purine-6-ylamine(9-propyladenine), a series of binding studies were undertakenusing solutions of mono-TP in CDCl3 solution,45 see ESI† fordetails (tetra-TP was found to be have insufficient solubility insuitable solvents to allow binding studies). The possibility ofself-association needs to be considered, particularly consid-ering the observed thymine/thymine interactions in the singlecrystal structure of mono-TP. Thus self-association bindingconstants were measured for mono-TP (Kd ¼ 6.1 � 3.0 M�1) and9-propyladenine (Kd ¼ 2.8 � 1.7 M�1) and found to be small inboth instances. In contrast the binding constant for the inter-action between mono-TP and 9-propyladenine was found to beK ¼ 91.8 � 20.5 M�1 indicating a favourable hetero-intermo-lecular interaction between the two species as anticipated. TheNMR studies also indicate both Hoogsteen46 and Watson–Crick47 binding modes between the thymine and adeninemoieties with corresponding shis in the C8–H and C2–Hproton signals respectively (Fig. 3).
Surface self-assembly studies
The surface self-assembly of tetra-TP and 9-propyladenine wasinvestigated at liquid–solid interfaces between highly oriented
Fig. 3 1H NMR spectrum illustrating shifts in the C8–H and C2–Hproton signals during titration of mono-TP with 9-propyladenine.Spectrum 1 corresponds to pure 9-propyladenine and spectra 2–17are recorded with constant mono-TP concentration and increasing9-propyladenine concentration.
Fig. 4 2D self-assembled network of tetra-TP at the TCB–HOPGliquid–solid interface. (a) Large scale STM image of the tetra-T-porphyrin (2.82 � 10�5 M) network at the TCB–HOPG liquid–solidinterface. The insert shows a high resolution, drift corrected STMimage of the network with an individual 2D unit cell marked in red: unitcell parameters a¼ 25.9� 0.5 A; b¼ 25.2� 0.6 A; and g¼ 91� 1�. STMimaging parameters: Vs ¼ �0.5 V; It ¼ 15 pA. Scale bar ¼ 20 nm (insert¼ 2 nm). (b) Molecular model of the tetra-TP network from MMsimulations. Unit cell parameters a ¼ 26.0 A; b ¼ 25.9 A; and g ¼ 90�.
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pyrolytic graphite (HOPG) and an organic solvent layer usingscanning tunnelling microscopy (STM). All attempts to adsorbmono-TP onto HOPG were unsuccessful which we attribute toweaker adsorption of this molecule on this surface under theconditions used, in part due to the enhanced solubility of thismolecule in comparison to tetra-TP and the smaller number ofpotential intermolecular hydrogen-bonding interactions.
Mixtures of tetrahydrofuran (THF) and 1,2,4-tri-chlorobenzene (TCB) with a 1 : 9 volume ratio respectively wereused as solvents for the tetra-TP and 9-propyladenine. Solutionsin the concentration range 5 � 10�6 to 5 � 10�5 M for tetra-TPand 8 � 10�6 to 1 � 10�3 M for the 9-propyladenine wereemployed. Even at these low concentrations the tetra-TP solu-tions were saturated with some visible undissolved material. Itwas found that saturated solutions were required in order toform self-assembled surface structures. A small quantity ofsaturated solution was deposited on to a preheated HOPGsubstrate held at 60 �C. Aer 5 minutes held at this temperaturethe substrate was allowed to cool to room temperature beforeSTM imaging.
STM images of tetra-TP self-assembled structures clearlydisplay the cruciform shape and symmetry of the tetra-TPmolecules suggesting they adsorb in a planar fashion on theHOPG substrate (Fig. 4a). The self-assembled structure displayslarge domains (>100 nm) of a hydrogen bond stabilised networkwith a P2 plane symmetry group. Analysis of dri corrected STMimages (Fig. 4a insert) provide 2D unit cell dimensions for thisstructure of a¼ (25.9� 0.5) A; b¼ (25.2� 0.6) A and g¼ (90� 2)�
and angles between the unit cell vectors and underlying HOPGsymmetry axes of a ¼ (6 � 3)�; and b ¼ (25 � 3)�. For details ofthe dri correction process applied to the STM images see theESI.†
Fig. 4b shows a structural model derived from molecularmechanics (MM) simulations for the tetra-TP network. Thismolecular structure is stabilised by thymine–thymine hydrogenbond dimers formed between adjacent tetra-TP molecules, in asimilar fashion to the bonding arrangement observed in thesingle crystal structure of mono-TP. The inter-thymine
hydrogen-bonding arrangement is also similar to that observedfor thymine adsorbed at the liquid–solid interface between1-octanol and HOPG.32 Additionally tetrakis(4-carboxy-phenyl)-porphyrin molecules adopt a similar hydrogen bond stabilisedstructure on an Au(111) surface37c where the network is stabi-lised via carboxylic acid hydrogen bond dimers. The MMsimulations were carried out with the molecules placed above asingle xed layer of graphene; further details of the simulationsare available in the ESI.†Unit cell dimensions andmeasured fromgeometry optimised structures produced values of a ¼ 26.0 A;b ¼ 25.9 A and g ¼ 91� and angles between the unit cell vectorsand underlying graphite symmetry axes of a ¼ 4.7�; and b ¼24.3�, in good agreement with the experimentally determinedvalues.
The asymmetric nature of the thymine groups decorating thetetra-TP molecules indicates that the molecules have thepotential to adopt a chiral arrangement when adsorbed onto asurface.48 Numerous previous studies have shown that prochiralmolecules tend to assemble into homochiral domains onsurfaces containing molecules of only a single handedness.49
The almost perfectly square 2D unit cell observed for the tetra-TP structure suggests that all of the thymine groups within anindividual tetra-TP molecule display the same orientation withrespect to the porphyrin core, and that individual domainscontain only molecules of a single handedness. The overallsurface structure of tetra-TP remains globally achiral by formingan equal area of mirror domains containing either right, or le-handedmolecules. STM images showing themirror image tetra-TP domain to that observed in Fig. 4 are shown in the ESI.†There is no signicant barrier to rotation of the thymine groupsprior to surface adsorption and although we see no evidence ofporphyrin molecules with mixed thymine orientations wecannot rule out the possibility of such orientations existing insmall localised regions.
The surface self-assembly of mixtures of tetra-TP and9-propyladenine on HOPG produced amolecular network with a
combination of disordered regions interspersed with smalldomains of an ordered co-crystal structure containing bothtetra-TP and 9-propyladenine. Fig. 5a shows a large scale STMimage displaying both the disordered structure (top right handcorner) and the ordered co-crystal (top le hand corner). Anal-ysis of high resolution dri corrected STM images for the tetra-TP–9-propyladenine co-crystal (Fig. 5a insert) produce unit celldimensions of a¼ (25.4� 0.7) A; b¼ (22.2� 0.9) A and g¼ (83.0� 2.0)� and angles between the unit cell vectors and underlyingHOPG symmetry axes of a ¼ (4.4 � 1.2)�; and b ¼ (26.9 � 2.1)�.
Using the same method as adopted for the tetra-TP networkMM simulations were performed to produce a molecular modelfor the tetra-TP and 9-propyladenine co-crystal network(Fig. 5b). This structure consists of a pair of 9-propyladeninemolecules surrounded by 4 tetra-TP molecules. The 9-propyla-denine molecules are linked to each other in a dimerichydrogen bonded arrangement and each 9-propyladenine isfurther linked to the surrounding tetra-TP molecules by threehydrogen bonds. The dimeric arrangement of the two 9-propy-ladenine molecules is the same as that calculated to be the moststable arrangement and previously observed for unfunctional-ised adenine adsorbed on HOPG.32 Two of the hydrogen bondslinking the 9-propyladenine to one of the neighbouring tetra-TPmolecules adopt a Watson–Crick47 binding mode. The remain-ing available hydrogen-bonded site on 9-propyladenine, the N3position, adopts a conventional N–H/N hydrogen bond to afurther tetra-TP molecule. Unit cell dimensions measured fromgeometry optimised MM simulations gave values of a ¼ 26.0 A;b ¼ 21.5 A; and g ¼ 85.7�. The angles between the unit cellvectors and underlying graphite symmetry axes were a ¼ 4.7�;and b ¼ 30.0�, in good agreement with the experimentallydetermined values. Similarly to the tetra-TP network, the tetra-TP and 9-propyladenine co-crystal also displays the formation ofhomochiral domains in which the thymine groups of the chiraltetra-TP molecule all adopt the same orientation with respect to
Fig. 5 2D self-assembled network of tetra-TP and 9-propyladenine atthe TCB–HOPG liquid–solid interface. (a) Large scale STM image ofthe tetra-TP (2.82 � 10�5 M) and 9-propyladenine (4.52 � 10�4 M)network at the TCB–HOPG liquid–solid interface. The insert shows ahigh resolution, drift corrected STM image of the network with anindividual 2D unit cell marked in red: unit cell parameters from STMimages a ¼ 25.4 � 0.7 A; b ¼ 22.2 � 0.9 A; and g ¼ 83 � 2�.STM imaging parameters: Vs ¼ �0.5 V; It ¼ 15 pA. Scale bar ¼ 200 A(insert ¼ 16 A). (b) Molecular model of the tetra-T-porphyrin networkfrom MM simulations. Unit cell parameters from MM simulations a ¼26.0 A; b ¼ 21.5 A; and g ¼ 85.7�.
1566 | Chem. Sci., 2015, 6, 1562–1569
the porphyrin core. An STM image displaying two mirror imagedomains of the tetra-TP and 9-propyladenine co-crystal isprovided in the ESI.†
The domain size for the tetra-TP and 9-propyladenine co-crystal and the prevalence of the disordered structure are linkedto both the concentration and ratio of the individual compo-nents. Fig. 6a–d shows four example STM images of structuresproduced using different concentrations and molar ratios oftetra-TP and 9-propyladenine. Fig. 6a has the same molar ratioof tetra-TP to 9-propyladenine, 1 : 16, as Fig. 5a but half theoverall molar concentration. The domain size for the tetra-TPand 9-propyladenine co-crystal has increased at the expense ofthe disordered arrangement.
Fig. 6b has the same molar concentration of tetra-TP asFig. 5a but a greatly reduced concentration of 9-propyladenineso that in this case the molar ratio of tetra-TP to 9-propylade-nine is 4 : 1. In this case the tetra-TP and 9-propyladenine co-crystal structure is not present; instead we observe the disor-dered network interspersed with domains of the tetra-TP mono-component network. As the amount of 9-propyladenine in themixture is increased domains of the tetra-TP and 9-propylade-nine co-crystal reduce in size until we are le with primarily thedisordered structure. This effect can be seen in Fig. 6c and dwhich show example STM images of mixtures with molar ratiosof tetra-TP to 9-propyladenine of 1 : 64 and 1 : 383 respectively.
Fig. 6 Influence of the concentration and molar ratio of componentson the morphology of the tetra-TP–9-propyladenine network. (a)Tetra-TP (1.41 � 10�5 M) and 9-propyladenine (2.26 � 10�4 M)mixture: molar ratio 1 : 16. (b) Tetra-TP (3.03 � 10�5 M) and 9-pro-pyladenine (8.07 � 10�6 M) mixture: molar ratio 4 : 1. (c) Tetra-TP(1.77 � 10�5 M) and 9-propyladenine (1.13 � 10�3 M) mixture: molarratio 1 : 64. (d) Tetra-T-porphyrin (5.04 � 10�6 M) and 9-propylade-nine (1.93 � 10�3 M) mixture: molar ratio 1 : 383. STM imagingparameters: Vs¼�0.5 V; It ¼ 15 pA. Scale bars: (a) 40 nm; (b) 10 nm; (c)20 nm; (d) 10 nm.
The delicate dependence of the co-crystal structure on theconcentration and molar ratio of the components suggests thatkinetic effects play a critical role in the self-assembly of tetra-TPand 9-propyladenine.
Conclusions
We have demonstrated a successful strategy for employing tet-ratopic tectons in the preparation of surface-based self-assem-bled arrays. Inter-thymine hydrogen bonds successfully lead tothe formation of square-grid lattices upon deposition of tetra-TP onto an HOPG substrate. The preferential formation ofheteromolecular hydrogen-bonds exhibited by thymine andadenine decorated tectons, validated in solution by NMRstudies, can be exploited to disrupt the homomolecular tetra-TParray and leads to the formation of a co-crystal between tetra-TPand 9-propyladenine. Studies of the self-assembly of tetra-TPand 9-propyladenine demonstrate a strong dependence onoverall concentration and molar ratio of components. Loweroverall concentrations lead to a greater proportion of ordereddomains and the degree of order in the resulting structures ishighly dependent on the molar ratio of tetra-TP : 9-propylade-nine. These studies indicate the importance of kinetic effects insurface self-assembly processes.
The system reported herein illustrates a successful strategythat has wide implications for the design of new tectons to beused in the formation of heteromolecular arrays. We demon-strate that the exploitation of the familiar recognition pathwaysof DNA bases can be incorporated into complex molecules andused to generate heteromolecular arrays. This approach has far-reaching implications for molecule/tecton design for preparingspecic hydrogen-bonded arrays on surfaces and stronglyindicates the possibility of preparing complex multicomponentarrays using a self-assembly stratagem.
Experimental
Details of the synthetic procedures employed are described inthe ESI.† The following intermediate species were synthesisedfollowing literature methods: 3,5-di-tert-butylbenzoic acid,50 3,5-di-tert-butylbenzaldehyde,51 3,5-di-tert-butylphenyl-dipyrro-methane,52 3-benzoylthymine39 and 9-propyladenine.53
CCDC-1034260 (mono-TP) and CCDC-1034261 (benzoyl-mono-TP) contain the ESI† for this paper. Further details of thesingle crystal structure renements are given in ESI† but detailsof the nal renement parameters are as follows for the struc-tures of mono-TP and benzoyl-mono-TP.
Crystal data for mono-TP
C74H86N6O3 (M ¼ 1107.48): triclinic, space group P�1 (no. 2), a ¼9.8270(6) A, b ¼ 16.9659(8) A, c ¼ 21.6298(13) A, a ¼ 69.273(5)�,b ¼ 81.505(5)�, g ¼ 87.976(5)�, V ¼ 3335.2(4) A3, Z ¼ 2, T ¼120(2) K, m(synchrotron) ¼ 0.063 mm�1, Dcalc ¼ 1.103 g mm�3,31 197 reections measured, 11 500 unique (Rint ¼ 0.0549)which were used in all calculations. The nal R1 was 0.0756(I > 2s(I)) and wR2 was 0.2097 (all data).
Crystal data for mono-phenyl(benzoylthymine)-tri-(3,5-di-tert-butylphenyl)porphyrin 6CHCl3
C86H92Cl18N6O3 (M ¼ 1895.75): triclinic, space group P�1 (no. 2),a ¼ 15.4721(5) A, b ¼ 16.3717(5) A, c ¼ 19.0278(6) A, a ¼92.068(2) �, b ¼ 110.731(3) �, g ¼ 99.280(3)�, V ¼ 4426.0(3) A3,Z ¼ 2, T ¼ 120(2) K, m(Cu-Ka) ¼ 0.063 mm�1, Dcalc ¼ 1.422 gmm�3, 52 160 reections measured, 15 638 unique (Rint ¼0.079) which were used in all calculations. The nal R1 was0.1220 (I > 2s(I)) and wR2 was 0.3846 (all data).
Details of the STM experiments, image analysis and MMsimulations can be found in the ESI.†
Acknowledgements
We would like to gratefully acknowledge the support of Engi-neering and Physical Sciences Research Council (EP/H010432/1).YH gratefully acknowledges receipt of a UCL Overseas ResearchScholarship (UCL-ORS). NRC gratefully acknowledges receiptof a Royal Society Wolfson Merit Award. MOB gratefullyacknowledges the Marie Skłodowska-Curie actions researchfellowship programme for the award of a Career IntegrationGrant (618777-PHOTOSURF).
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